For over 90 years, annuli between an oil well casing and the surrounding borehole have been cemented to prevent vertical communication of formation fluids inside the borehole. Particular difficulties occur in cementing wellbores when the wellbore penetrates salt or potash formations. Cement slurries that are not salt saturated will dissolve a portion of the formation as the cement slurry sets leaving voids and poor bonding between the cement and the salt formation. Salt saturated water is therefore used in the preparation of cement slurries for cementing within salt and potash formations. Salts generally retard setting of Portland type cements, and reduce the effectiveness of many common cement additives for Portland cement slurries. The set cements also do not have as high of a compressive strength of Portland cements prepared from fresh water. Additional setting accelerators and additives are therefore required that increase the cost of such cements. Such salt water Portland cement slurries are therefore relatively expensive. Such salt water Portland cement slurries can also be difficult to formulate due to interactions between the salt and the many cement additives required.
Cementing wellbores through saltdome formations is particularly difficult because the salt formation will slowly flow. The casing must therefore be particularly strong to resist movement of the salt formation. Strength of cement around the casing typically cannot be relied upon to resist movement of the salt formation due to typically poor bonding between the cement and the casing. If such bonding could be improved, the cement could add considerable strength to the casing.
It is an object of this invention to provide a method to cement a casing in a wellbore, the method utilizing an improved salt water cement slurry.
It is a further object of this invention to provide such a method wherein the cement slurry is prepared from drilling fluids.
It is yet a further object of this invention to provide a method for cementing a wellbore within a salt formation resulting in good bonding between the salt formation and the set cement.
In a preferred embodiment, it is an object of this invention to provide a method for cementing a casing in a wellbore in a salt formation slurry that will result in good bonding both between the salt formation and the cement and the cement and the casing.